Numerous experiments over the last decades demonstrate that caloric restriction increases longevity, but little evidence exists relative to direct efffects on organ function. Male Wistar rats had food withheld every other day for 6 weeks to study at 24, 30 mo. (EOD24, EOD30) and were compared to normal lifespan 24 mo. controls (C). C are unattainable at 30 mo. Body weights (BW, gm.) were (C= 504 +/- 15; EOD24 = 354 +/- 10; EOD30 = 349 +/- 9), dry left ventricular weight, (LV, gm.), relative to body size (indexed by tibia length; mm. (TL) as TL/LV) were (C = .0597 +/- .001; EOD24 = .037 +/- .001; EOD30 = .042 +/- .002) or indexed by BW as LV/BW (C = .566 +/- .03; EOD24 = .456 +/- .01; EOD30 = .53 +/- .02) were all P less than .001 versus C except EOD30 LV/BW, NS. Isolated isometric LV trabecular performance ([Ca++] = 2.5 mM, 29 C, Lmax) was not statistically significantly altered by food restriction even for 30 mo. (All P less than NS versus C.) C, 10N, 2.4 +/- .3Act Force, 156 +/- 6TPT, 268 +/- 11CD, 112 +/- 5RT 1/2, 34 +/- 4DF/DT; EOD24, 25N, 3.2 +/- .4 Act Force, 147 +/- 3TPT, 250 +/- 7CD, 103 +/- 4RT 1/2, 45 +/- 6DF/DT; EOD30, 6N, 3.3 +/- .9Act Force, 146 +/- 4TPT, 254 +/- 9CD, 108 +/- 5RT 1/2, 48 +/- 13DF/DT. Thus EOD causes marked atrophy of the heart, but does not significantly alter in vitro isometric contraction. Relative heart size per se produced by caloric restriction does not have a direct effect of myocardial performance.